1. Field of the Invention
This invention relates to an apparatus for detecting acoustic signals; in particular, underwater acoustic signals.
2. Description of the Prior Art
Hydrophone receivers are used for detecting underwater acoustic signals. These signals may emanate from natural sources (marine life), from ships at sea (both surface and subsurface), and from the Earth, itself (seismic signals). Hydrophones are used for locating and identifying the sources of these emissions by sonar techniques, both in the passive and in the active mode. In the passive mode, the hydrophone senses signals generated by the object of interest, itself; in the active mode, the hydrophone senses echo signals resulting from reflections off the object of signals that are generated by the investigator. In some cases of active sonar investigations, the outgoing signal is generated by the hydrophone. In other cases, a separate device (either continuous or explosive) is utilized for generation.
The sensing element of a hydrophone is an acoustic transducer; i.e., a device for converting an acoustic signal to a signal in another form, such as an electrical signal. Among the acoustic transducer materials known in the art are piezoelectric and magnetostrictive materials.
Piezoelectric materials develop an electrical signal when stressed and are thus suitable for use as acoustic transducers. Piezoelectric materials that have been used for that purpose include ceramics such as lead zirconate titanate. A problem with piezoceramic acoustic transducers is that they tend to be brittle. Certain applications require hydrophones that are a meter or more in length and that must be able to withstand appreciable bending stress. To accomplish this with ceramics, it is necessary to combine many relatively short cylinders that are mechanically free to move independently (i.e., to flex like a spine made of rigid vertebrae) but that are electrically coupled so that their individual output voltages are summed. This permits the element to appear electrically as a long unit while providing the required mechanical flexibility. Since each element must have its own charge amplifier and the output characteristics of each element and amplifier must be closely matched for good performance, these ceramic hydrophones tend to be rather complex.
Magnetostrictive materials that have been used in acoustic transducers include nickel, nickel alloys, and rare-earth iron alloys. Nickel and its alloys have several limitations, including relatively low magneto-mechanical coupling factor, k. The efficiency of a transducer is directly proportional to k.sup.2. Some rare-earth iron alloys have high magnetostriction; however, these alloys typically have high magnetic anisotropy, which tends to make them undesirably magnetically hard, and have low mechanical ductility, which places severe restrictions on their use.
Magnetostriction of ferromagnetic metallic glasses was studied by O'Handley (Solid State Commun. 21, 1119 (1977)) He found that the magnetostriction of iron-cobalt glasses shows a maximum at low cobalt concentration while that of the iron-nickel glasses drops monotonically from its value at Fe.sub.80 B.sub.20. Arai and Tsuya (J. Appl. Phys. 49, 1718 (1978)) measured the magnetostriction and magneto-mechanical coupling coefficient, k, of iron-rich amorphous ribbon. They found values of k as large as 0.75 for amorphous Fe.sub.78 Si.sub.10 B.sub.12 ribbon after magnetic annealing.